Fasting is nothing new; humans have been practicing food restriction for spiritual, political, and health reasons for many years. Intermittent fasting has been in the spotlight recently as a strategy for weight management and improved metabolic health. So what is it, and how does it work?
Intermittent fasting is the practice of limiting calorie intake for a certain time period. It can take different forms. In alternate day fasting, an individual restricts all intake for one day and eats ad libitum (freely) the next. In modified fasting, he or she restricts energy intake on several scheduled days. This is the basis of diets such as the 5:2 Fast Diet, in which women limit their calories to 500 per day and men to 600 per day for two days of the week. In time-restricted feeding, eating is limited to a specific time window (e.g. from 11 am-6 pm). There are also religious fasts. During the month of Ramadan, Muslims traditionally fast from dawn until sunset, while Jews abstain from food on Yom Kippur.
What does the research say?
Human studies have shown that intermittent fasting can help people lose weight and improve their metabolic health by affecting inflammatory markers and blood levels of cholesterol, triglycerides, fasting insulin, and glucose. This is important because an improved cholesterol profile and lower triglycerides can help protect against heart attacks and strokes, whereas irregularities in insulin and glucose levels are associated with an increased risk of diabetes. Some scientists believe that fasting acts as a positive stressor to cause favorable metabolic adaptations at the cellular level. It’s a similar idea to exercising; just as your body responds to microscopic muscle tears by rebuilding muscles to be bigger and stronger, it could adapt to metabolic stress by becoming more efficient.
Intermittent fasting doesn’t necessarily mean that you eat less overall. One of the mechanisms by which fasting may improve health independently of total caloric intake is by aligning feeding times with natural circadian rhythms. In humans, circadian biology is primarily regulated by a tiny region of the brain within the hypothalamus called the suprachiasmatic nuclei (SCN), which is responsive to light and dark cues. We also have regulating tissues in organs such as the liver that respond to breakdown products from our food. These biological clocks influence metabolism, meaning that we burn fuel more efficiently at some times of the day than others. They also impact physiological processes such as digestion. Desynchronizing these regulators (e.g. by eating most of your calories at night) can impair nutrient utilization and increase risk for obesity and chronic disease.
Practical applications
Some researchers believe that fasting can slow metabolic rate and that small frequent meals are better for weight loss and maintenance. However, the research on the relationship between meal frequency and metabolism has produced mixed results, and there is no universal agreement within the scientific community about the best strategy. What makes sense for one person might not work for another. Jessie Wrye, a registered dietitian in Napa, California, has seen this first hand. “What I really see missing in most of the patients that are referred to me is a lack of routine — any routine — along with poor habits,” she says. “Intermittent fasting or small frequent meals would both offer structure and balance to the diet, so it comes down to personal preference. When I looked at the research for intermittent fasting I found a lot of positive results, but to implement the practice into daily life can be a big challenge depending on the person.”
William Giuffre, a physical therapist and former bodybuilder from Boston, Massachusetts, has been a strong proponent of intermittent fasting for several years. He currently practices a form of time-restricted feeding in which he typically goes without food for 18-23 hours. “I had never fasted before and I was very skeptical,” he says. “I just kept hearing more and more about the benefits of fasting.” He typically has his first meal of fruit, nuts and seeds several hours after waking and tries not to eat late at night. Since doing this, he says that he has felt that his energy levels are more sustained and that he has fewer cravings. “The first thing you notice after doing it for a few days is you don’t get terribly hungry anymore. Of course, hunger still comes, but you’re not ravenous. Eating is more of an option than a necessity. I’ve lost approximately 25 to 28 pounds in a year from doing this diet but I really don’t feel like it’s an effort at all.”
Final thoughts
Ultimately, the way that you time your meals should align with your health goals and lifestyle. Personally, I prefer to eat small meals consisting of fruits, vegetables, protein, complex carbohydrates and healthy fats throughout the day. I also try to stop eating several hours before I go to bed — it helps me sleep better and gives me more energy the next day. Intermittent fasting is not a good idea for everyone. It should not be undertaken by pregnant women or children, and can cause dangerous blood sugar swings in people with diabetes. However, the research shows that properly executed intermittent fasting regimens may improve health outcomes in healthy adults.
References
Bass, Joseph, and Joseph S. Takahashi. “Circadian integration of metabolism and energetics.” Science 330.6009 (2010): 1349-1354.
Challet, Etienne. “Circadian clocks, food intake, and metabolism.” Progress in Molecular Biology and Translational Science 119 (2013): 105-135.
Longo, Valter D., and Mark P. Mattson. “Fasting: molecular mechanisms and clinical applications.” Cell Metabolism 19.2 (2014): 181-192.
Patterson, Ruth E., et al. “Intermittent fasting and human metabolic health.” Journal of the Academy of Nutrition and Dietetics 115.8 (2015): 1203-1212.
Thaiss, Christoph A., et al. “Transkingdom control of microbiota diurnal oscillations promotes metabolic homeostasis.” Cell 159.3 (2014): 514-529.
Schoenfeld, Brad Jon, Alan Albert Aragon, and James W. Krieger. “Effects of meal frequency on weight loss and body composition: a meta-analysis.” Nutrition Reviews 73.2 (2015): 69-82.
Sutton, Elizabeth F., et al. “No evidence for metabolic adaptation in thermic effect of food by dietary protein.” Obesity 24.8 (2016): 1639-1642.